Herpes simplex virus is a useful model for studying the mechanisms involved in DNA replication in eukaryotic cells. Our current efforts are directed toward studying this process with the purified protein products for the seven viral genes that are necessary to carry out authentic origin-dependent DNA replication in infected cells. We are currently using both biochemical and molecular genetic approaches to understand the function of these polypeptides in detail. The HSV DNA polymerase consists of a stable complex of two polypeptides: UL30, the catalytic subunit, and UL42, an accessory subunit. Several lines of evidence support the view UL42 increases the efficiency of the DNA polymerase by increasing its processivity. We have begun to dissect the function of the UL42 processivity factor using directed mutagenesis. Our results to data reveal that the carboxyl terminal third of the polypeptide is not essential for any function in DNA replication. Additional mutants that may shed some light on the mechanism of UL42 and the regulation of processivity are currently under investigation. The UL5, UL8, and UL52 polypeptides form a three protein complex that has both helicase and primase activities. We have proposed that UL52 has the active site for primase function, and are trying to obtain direct support for this view. Several mutants of UL52 that have missense mutations in the most highly conserved regions of the protein have been isolated. At least three of these mutants do not support DNA replication, even though the mutant polypeptide is capable of forming a heterotrimeric complex with UL5 and UL8. The biochemical defect of these mutants is now being determined.